Few microorganisms are as versatile as E. coli. As well as being an important member of the normal intestinal microflora of mammals, it has been widely exploited as a host in recombinant DNA technology. In addition, however, E. coli can also be a deadly pathogen.
E. coli strains have traditionally been classified as either commensal or pathogenic, and pathogenic strains are then sub-classified as intestinal or extraintestinal strains. Classification may also be based on the ‘K’ antigens. The best-studied ‘K’ antigen is ‘K1’, which is considered to be the major determinant of virulence among those strains of E. coli that cause neonatal meningitis. The K1 antigen is a homopolymer of α-2,8-linked sialic acid, like the capsular saccharide of serogroup B meningococcus.
More recent taxonomic techniques such as multilocus enzyme electrophoresis (MLEE) classify E. coli into five phylogenetic groups (A, B1, B2, D & E), and these groupings do not match the traditional ones. For instance, MLEE group B1 includes both commensal and pathogenic strains, and group D includes both intestinal and extraintestinal strains.
The extraintestinal pathogenic strains (or ‘ExPEC’ strains [1]) of E. coli fall into MLEE groups B2 and D, and include both uropathogenic (UPEC) strains and meningitis/sepsis-associated (MNEC) strains. UPEC strains cause urinary tract infections (UTIs), and are a common cause of cystitis. They also cause pyelonephritis (and its complications such as sepsis) and catheter-associated infections.
MNEC strains cause neonatal meningitis (0.1 cases per 1000 live births) with case fatality rates ranging from 25 to 40%, and are also responsible for around ⅙ of sepsis cases.
Most previous ExPEC vaccines have been based on cell lysates or on cellular structures. SOLCOUROVAC™ includes ten different heat-killed bacteria including six ExPEC strains, and a successful phase II clinical trial was reported in reference 2. URO-VAXOM™ is an oral tablet vaccine containing lyophilised bacterial lysates of 18 selected E. coli strains [3]. Baxter Vaccines developed a UTI vaccine based on pili from 6 to 10 different strains, but this product has been abandoned. MedImmune developed a product called MEDI 516 based on the FimH adhesin complex [4], but phase II clinical trials shows inadequate efficacy. Moreover, there was a risk with this vaccine that it would also affect non-pathogenic FimH+ve strains in the normal intestinal flora, and it was expected that it would be effective against UPEC strains only, because of its bladder-specific adherence mechanism, leaving other ExPEC strains uncontrolled.
There is thus a need for improved ExPEC vaccines, including a need to move away from crude cell lysates and towards better-defined molecules, and a need to identify further antigens that are suitable for inclusion in vaccines, particularly antigens that are prevalent among clinical ExpEC strains without also being found in commensal strains. Within the ExPEC group, there is a particular need to identify antigens suitable for immunising against MNEC strains.
One way of addressing these needs was reported in reference 5, where the inventors looked for genes present in genomes of MLEE types B2 and D but absent from MLEE types A and B1. Further comparative approaches, based on subtractive hybridisation, have been reported in references 6 & 7. Virulence genes in ExPEC strains have also been identified in reference 8. Reference 9 discloses an analysis of four pathogenicity islands in UPEC E. coli strain 536.
Reference 10 used the genome sequence of UPEC (O6:K2:H1) strain CFT073 [11,12] to identify sequences not present in non-pathogenic E. coli strains. Reference 13 discloses a comparison of the genome sequence of E. coli human pyelonephritis isolate 536 (O6:K15:H31), an UPEC, with sequence data for strains CFT073 (UPEC), EDL933 (enterohemorrhagic) and MG1655 (non-pathogenic laboratory strain). Genome sequences of pathogenic strains are available in the databases under accession numbers AE005174, BA000007 and NC-004431. A sequence from a non-pathogenic strain is available under accession number U00096.
It is an object of the invention to provide further antigens for use in immunisation against pathogenic E. coli strains, particularly against ExPEC strains, and more particularly against MNEC strains.